This invention relates to drop-on-demand ink jet printing systems and more particularly, to a thermal ink jet printer having a rotatable platen having circuitry mounted therein including a resonant vibratory device on which the ink droplets ejected from the printhead nozzles are received and the droplet mass measured.
Thermal ink jet printing is generally a drop-on-demand type of ink printing system which uses thermal energy to produce a vapor bubble in an ink filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electric pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubbles grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink in the channel between the nozzle and the bubble starts to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line towards a recording medium, such as paper.
Thus, thermal ink jet devices operate by pulsing heating elements in contact with ink so that bubbles are nucleated, ejecting ink droplets toward the paper. It has been found during print tests that print quality is affected as the device heats up. This is because the volume of the droplet and therefore the printed spot or pixel increases as a function of printhead temperature. Through study of this problem, it has been found that both the mass and velocity of the droplet increase with device temperature, and that both the mass and velocity contribute to increase pixel size on the paper. For the carriage type ink jet printer with sufficiently high printing density, the spot size increases as the carriage traverses the page. Then, as it pauses at the end of travel and reverses direction, it cools slightly, so that the next line or swath printed on the way back has increasing pixel sizes in the opposite direction. This gives rise to light and dark bands, which are most pronounced at the edges of the paper. Similarly, other patterns of high and low density printing are degraded by undesired change in pixel size with device temperature.
U.S. Pat. No. 4,788,466 to Paul et al discloses a Q-loss compensation apparatus for a piezoelectric sensor such as a quartz crystal microbalance or other vibratory device wherein the vibration amplitude of the device is controlled by negative feedback in a manner to obviate the effect of of energy loss associated with viscous damping of a large liquid drop on the quartz crystal face serving as an environment for an experiment to measure mass deposited on the crystal. The specific apparatus includes an oscillator circuit for the vibratory device in which two similar variable gain amplifiers provide the regenerative feedback for maintaining oscillation. The negative feedback amplitude control circuit serves to maintain constant the output from the variable gain amplifier following the quartz crystal in the oscillator loop, and it thus of a near constant value equal to the product of the crystal vibration amplitude and the square root of the total gain in the oscillator loop. This results in stable operation of the quartz crystal with little influence from changing conditions such as temperature, viscosity of the fluid, evaporation of the fluid, etc., at the same time producing a linear frequency change dependent on the quantity of mass deposited on the crystal face from the liquid environment. Frequency change is measured in a conventional manner with accuracy of about one part per ten million, thereby permitting determination of minute mass amounts on the order of one nanogram.
U.S. Pat. No. 5,036,337 to Rezanka discloses a method and apparatus for controlling the volume of ink droplets ejected from thermal ink jet print heads. The electrical signals applied to heating elements for generating droplet ejecting bubbles thereon are composed of packets of electrical pulses. Each pulse and spacing there between are varied in accordance with one or more whole, clock or timing units. The number of pulses per packet and width of pulses and spacing there between are controlled in accordance with the manufacturing tolerance variations, the location of the addressed heating heating element in the printhead, the number of parallel heating elements concurrently energized, and optionally the temperature of the printhead in the vicinity of the heating elements to maintain the desired volume of the ejected droplets.
U.S. Pat. No. 5,107,276 to Kneezel et al discloses a thermal ink jet printer which has a printhead that is maintained at a substantially constant operating temperature during printing. Printing on demand is accomplished by the ejection of ink droplets from the printhead nozzles in response to energy pulses selectively applied to heating elements located in ink channels upstream from the nozzles which vaporize the ink to form temporary bubbles. To prevent printhead temperature fluctuations during printing, especially in translatable carriage printers, the heating elements not being used to eject droplets are selectively energized with energy pulses having insufficient magnitude to vaporize the ink.